Heat-resistant methacrylic copolymers, production process thereof and optical elements

ABSTRACT

A process for production of methacrylic copolymers, characterized by charging a monomer mixture comprising from 70 to 95% by weight of methyl methacrylate, from 0 to 15% by weight of α-methylstyrene, from 0 to 20% by weight of styrene and from 2 to 15% by weight of maleic anhydride at a molar ratio of the sum of α-methylstyrene. and styrene to maleic anhydride laying within the range of 1.0 to 2.5 into a polymerizing vessel containing at least one polymeric membrane layer having an oxygen permeability of 1.5×10 −13  (mol·m −2 ·s −1 ·Pa −1 ) or below, sealing the polymerizing vessel, and polymerizing the monomer mixture under heating in a gas phase; a methacrylic copolymer obtained by the production process which is reduced yellowing and shows excellent transparency and heat resistance; and an optical element composed of the methacrylic copolymer.

TECHNICAL FIELD

[0001] The present invention relates to a heat-resistant methacryliccopolymer, a process of producing the same, and an optical element.

BACKGROUND ART

[0002] Generally, methacrylic resins containing methyl methacrylate asthe main component are used in various fields such as illuminationcovers, automobile parts, sign boards, ornaments, general merchandisesand the like, due to their heat resistance and remarkable transparency.However, the heat resistance of methacrylic resins is as insufficient asabout 100° C., and application development thereof is restricted inconsiderable fields, consequently, improve in heat resistance isdesired. Therefore, studies are conducted widely to improve the heatresistance of methacrylic resins, and there are some reports andsuggestions.

[0003] For example, there are suggested a copolymer of methylmethacrylate, α-methylstyrene and maleic anhydride (JP-A No. 4-300907),a terpolymer of methyl methacrylate, styrene and maleic anhydride (JP-ANo. 4-227613), a quaternary polymer of methyl methacrylate,α-methylstyrene, styrene and maleic anhydride (JP-A No. 61-271313), andthe like.

[0004] As the method of polymerizing these ternary and quaternarypolymers, a bulk polymerization method, solution polymerization method,suspension polymerization method and emulsion polymerization method areexemplified.

[0005] The emulsion polymerization method has a problem that since anemulsifier and a salting agent are used in the production process, thesesubstances remain in a copolymer, and the resulted copolymer has loweredtransparency and color tone. In the emulsion polymerization method andsuspension polymerization method, since maleic anhydride is polymerizedin water, maleic anhydride turns into maleic acid and production of acopolymer containing a maleic anhydride unit becomes difficult. Further,the solution polymerization method requires a solvent removal process,namely, is disadvantageous in cost.

[0006] On the other hand, the bulk polymerization method is advantageousin cost as compared with the solution polymerization method. As thisbulk polymerization method, a continuous bulk polymerization method anda casting polymerization method are general. However, in the continuousbulk polymerization method, a large scale apparatus is necessary causingan enormous cost for the facility, and production of multiple productsin small amounts is difficult. In the case of the casting polymerizationmethod, general is a cell casting method in which raw materials such asmonomers and the like are poured into glass cells or metal cells such asstainless and the like and the raw materials are heated in an aqueousphase or gas phase, however, when maleic anhydride is used, there areproblems that peeling of a polymerized product from cells made of glass,metal and the like after polymerization is difficult, meaning poorworkability, and the like. Further, the heating mode in an aqueous phasegenerates large cost necessary for the facility. Though the heating modein a gas phase is advantageous in cost, when the thickness of a cellincreases, heat removal in polymerization is small due to low heatconductivity of air, and heat generation in polymerization cannot besuppressed to cause a tendency of burst of a polymerization reaction,decrease in transparency of a polymerized product due to temperaturedisproportionation occurs easily, and other problems are caused.

[0007] For the purpose of improving heat removal from raw materials inthis polymerization, there is also a method, for example, in which amonomer mixture is cast into a polymerizing vessel made of a thinmaterial such as a nylon polymer film and the like and heated in a gasphase, to suppress burst of a polymerization reaction and temperaturedisproportionation in a polymerizing vessel. However, in this method,yellowing of a polymerized product of raw materials containing maleicanhydride is often remarkable. The copolymer showing remarkableyellowing is not easily utilized in optical elements and the like.

DISCLOSURE OF THE INVENTION

[0008] The present invention has been accomplished in view of theabove-mentioned problems in conventional technologies and the objectthereof is to provide a methacrylic copolymer which is polymerized underheating in a gas phase, reduced in yellowness, and excellent intransparency and heat resistance, a process of producing the same, andan optical element composed of a methacrylic copolymer.

[0009] The present inventors have intensively studied to attain theabove-mentioned object, and resultantly found that yellowing afterpolymerization of a copolymer containing maleic anhydride can beimproved by using a material having oxygen permeability of not more thanspecific value as a material of polymerizing vessel.

[0010] Namely, the present invention is a process for production ofmethacrylic copolymers, characterized by charging a monomer mixturecomprising from 70 to 95% by weight of methyl methacrylate, from 0 to15% by weight of α-methylstyrene, from 0 to 20% by weight of styrene andfrom 2 to 15% by weight of maleic anhydride at a molar ratio of the sumof α-methylstyrene and styrene to maleic anhydride laying within therange of 1.0 to 2.5 into a polymerizing vessel containing at least onepolymeric membrane layer having an oxygen permeability of 1.5×10⁻¹³ (molm⁻²·s⁻¹·Pa⁻¹) or below, sealing the polymerizing vessel, andpolymerizing the monomer mixture under heating in a gas phase.

BEST MODES FOR CARRYING OUT THE INVENTION

[0011] In the present invention, the monomer mixture subjected tocopolymerization comprises from 70 to 95% by weight of methylmethacrylate, from 0 to 15% by weight of α-methylstyrene, from 0 to 20%by weight of styrene and from 2 to 15% by weight of maleic anhydride,and has a molar ratio of the sum of α-methylstyrene and styrene tomaleic anhydride laying within the range of 1.0 to 2.5.

[0012] In the present invention, the methyl methacrylate (hereinafter,described as “MMA”) which is the main component constituting amethacrylic copolymer excellent in heat resistance is a componentnecessary for maintaining optical property, weather resistance andmechanical property inherent to a methacrylic resin, and used in anamount laying within the range of 70 to 95% by weight. Further, thisamount is preferably 80% by weight or more. When this amount is lessthan 70% by weight, the above-mentioned properties are lost, and whenover 95% by weight, heat resistance lowers.

[0013] The α-methylstyrene (hereinafter, described as “αMS”)constituting a copolymer is a component which improves thecopolymerization property of individual components in polymerizing amonomer mixture, and is one of components improving the heat resistanceof the resulting copolymer, being used in an amount laying within therange of 0 to 15% by weight. Further, this amount is preferably 3% byweight or more, more preferably 6% by weight or more, and preferably 12%by weight or less. The content of αMS is preferably controlled by thecontent of other monomer. By increase in this content, the heatresistance of the resulting copolymer can be increased. When thiscontent is over 15% by weight, mechanical property decreases, and theamount of remaining monomers in the resulted copolymer increases becauseof lowering of polymerization speed and deterioration of heat-resistantdecomposing property, further, the copolymer tends to be colored easilyor a coloring tendency is observed in molding a copolymer.

[0014] The styrene (hereinafter, described as “St”) constituting acopolymer is a component which promotes copolymerization of MMA, αMS andmaleic anhydride component. Though copolymerization property of MMA andmaleic anhydride is poor, copolymerization property is improved byadding St and copolymerizing them, and a copolymer containing a smallamount of unreacted monomers is obtained. By addition of St, the heatresistant decomposing property of the resulting copolymer can beimproved, and the molding property of the copolymer can also beimproved. However, when the addition amount of St is too large, the heatresistance of the resulting copolymer lowers, therefore, the additionamount is preferably controlled by the content of other monomer. St isused in an amount laying within the range of 0 to 20% by weight.Further, this amount is preferably 8% by weight or less.

[0015] The maleic anhydride (hereinafter, described as MAH) constitutinga copolymer is one of components improving, like αMS, the heatresistance of a copolymer, and used in an amount laying within the rangeof 2 to 15% by weight. Further, this amount is preferably 8% by weightor less. Though this amount is required to be 2% by weight or more forimparting sufficient heat resistance to a copolymer, when over 15% byweight, yellowing of a copolymer becomes remarkable, and waterabsorption coefficient also increases.

[0016] The amounts of αMS, St and MAH used in the present invention arewithin the above-mentioned ranges, and additionally, they are used at amolar ratio of the sum of αMS and St to MAH laying within the range of1.0 to 2.5. When this molar ratio is less than 1.0, yellowing of acopolymer becomes remarkable and appearance deteriorates. On the otherhand, when the molar ratio is over 2.5, the transparency of a copolymerdeteriorates, and the amount of unreacted monomers in a copolymerincreases, and deterioration in physical properties tends to be causedsuch as decrease in strength of a molded article of a copolymer, foamingin molding, and the like.

[0017] In the present invention, further other monomers can be added toa monomer mixture, in an amount not deteriorating the physicalproperties of the resulting copolymer. As the other monomers, acids suchas methacrylic acid, acrylic acid; acrylonitrile; maleimides such asN-phenylmaleimide, cyclohexylmaleimide; acrylates such as methylacrylate, butyl acrylate, cyclohexyl acrylate; methacrylates other thanMMA such as butyl methacrylate, cyclohexyl methacrylate, glycidylmethacrylate; polyethylene glycol di(meth)acrylates such as diethyleneglycol di(meth)acrylate, triethylene glycol di(meth)acrylate;1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, dipentaerythritol hexaacrylate andthe like are listed. Further listed are monomers having a polyfunctionalgroup such as saturated or unsaturated polyester poly(meth)acrylatescomposed of a condensate such as malonicacid/trimethylolethane/(meth)acrylic acid, succinicacid/trimethylolethane/(meth)acrylic acid and the like, and mixturesthereof.

[0018] In the process of production of the methacrylic copolymerexcellent in heat resistance, polymerization is conducted in a gasphase. In conducting polymerization in a gas phase, heat removal tendsto be insufficient, and foaming of the copolymerized substance composedof copolymer and burst of polymerization reaction due to polymerizationheat generation, and the like, may occur in some cases. When theresulted copolymer contains much foaming, it is sometimes difficult toremove the copolymer from the polymerizing vessel after polymerizationreaction. In this case, fragments of the polymerizing vessel aresometimes mixed in the resulted copolymer. In this case, opticalproperty may deteriorate in a molded article obtained by molding acopolymer formed by grinding a copolymerized material. Further, when apolymerization reaction bursts, there is a possibility of breakage of avessel itself in polymerization, causing a crisis. Then, if a terpenoidcompound is further added to a monomer mixture and these arepolymerized, polymerization peak temperature can only be lowered withscarce change of polymerization peak time, and foaming by polymerizationheat generation and burst of a polymerization reaction can be prevented.

[0019] As the terpenoid compound, compounds such as terpinolene,myrcene, limonene, α-pinene, β-pinene, α-terpinene, β-terpinene,γ-terpinene and the like are listed.

[0020] It is necessary to control the addition amount of a terpenoidcompound selected, depending on various conditions such as the additionamount of each monomer, the amount of a polymerization initiator,polymerization temperature and the like, since an effect of suppressingpolymerization peak temperature differs depending on the kind of aterpenoid compound. In general, this addition amount is preferably from0.0001 to 0.1 part by weight per 100 parts by weight of a monomer mixed.Further, this amount is more preferably 0.05 parts by weight or less.When the addition amount is too small, an effect of suppressingpolymerization peak temperature lowers. When the addition amount is toolarge, decrease in physical properties of a copolymer obtained byincreasing the amount of unreacted remaining monomers tends to becaused.

[0021] In the present invention, a polymerization initiator is usuallyused for polymerization of a monomer mixture. As the polymerizationinitiator, known compounds can be used. It is also possible to used aplurality of polymerization initiators together. For example, it is alsopossible to control time until polymerization completion, by combining aplurality of polymerization initiators having different 10 hour halflife temperatures.

[0022] The polymerizing vessel used in the present invention is requiredto have at least one polymeric membrane layer (hereinafter, described as“oxygen shielding membrane” having an oxygen permeability of 1.5×10⁻¹³(mol·m⁻²·s⁻¹·Pa⁻¹) or below. When the oxygen permeability is over1.5×10⁻¹³ (mol·m⁻²·s⁻¹·Pa⁻¹), the resulting copolymer tends to beyellowed. The oxygen permeability is preferably lower, and morepreferably 1.5×10⁻⁴ (mol·m⁻²·s⁻¹·Pa⁻¹) or below. This oxygenpermeability is measured according to the same manner as in JIS K7126method B excepting a temperature of 20° C. and a humidity of 0%.

[0023] The thickness of an oxygen shielding membrane is preferably 5 μmor more, more preferably 10 μm or more, in view of its strength. Fromthe standpoints of heat conductivity, handling easiness, processabilityinto a polymerizing vessel and the like, the thickness is preferably1000 μm or less, more preferably 500 μm or less.

[0024] The form of the polymerizing vessel is not particularlyrestricted provided that a monomer mixture can be poured into thepolymerizing vessel, preferably oxygen dissolved and remaining in amonomer mixture can be removed, and then, the polymerizing vessel can besealed. From the standpoint of easy handling, a polymerizing vessel inthe form of bag made of a material having an oxygen shielding membraneis preferable.

[0025] As the material of the oxygen shielding membrane, polyvinylalcohol, polyvinylidene chloride, ethylene-vinyl alcohol copolymer,polyacrylonitrile and the like are listed. When a commercially availablefilm is used as the oxygen shielding membrane, commercially availableproducts having trade names such as Eval (manufactured by Kuraray Co.,Ltd.), Vobron, Vobron SE (both manufactured by Nippon Synthetic ChemicalIndustry Co., Ltd.), Embler (manufactured by Unitika Ltd.) and the like,for example, can be used. When a material used for the oxygen shieldingmembrane is easily dissolved in a monomer mixture used in the presentinvention, it may be permissible that other material not being dissolvedis laminated on the surface side in contact with a monomer mixture, andthe resulted multi-layer structure sheet or multi-layer structure filmcontaining an oxygen shielding membrane is used. By multi-layerstructure, strength increases, easy processability is obtained, acopolymer is easily peeled after polymerization, and other merits areobtained. Specifically, multi-layer structure sheets or multi-layerstructure films are mentioned obtained by pasting a film ofpolypropylene, polyethylene, polyester, nylon and the like onto a filmof polyvinyl alcohol, polyvinylidene chloride, ethylene-vinyl alcoholcopolymer and the like.

[0026] The thickness of a material constituting the polymerizing vesselcontaining an oxygen shielding membrane is preferably 10 μm or more,more preferably 20 μm or more in view of its strength. From thestandpoints of heat conductivity, handling easiness of a polymerizingvessel, processability into a polymerizing vessel and the like, thethickness is preferably 1000 μm or less, more preferably 500 μm or less.

[0027] When the concentration of oxygen dissolved and remaining in theprepared monomer mixture is high, the resulted copolymer is yellowed,consequently, it is preferable that the concentration of oxygendissolved and remaining is as low as possible. Specifically, it ispreferably 10 ppm or less, more preferably 1 ppm or less. As the methodof removing oxygen dissolved and remaining, there are listed a method inwhich a monomer mixture is subjected to deairing under reduced pressurein a vacuum box, a method in which a nitrogen gas is directly blown intoa monomer mixture for decades of minutes, and other methods. Also, thewater content in the prepared monomer mixture is preferably lower.Further, also when a polymer obtained by grinding a polymerizedsubstance is made into a molded article such as pellets, opticalelements and the like by melt-extrusion, it is desirable to decrease theinfluence of oxygen. For example, a method of forming a molded articlesuch as pellets, optical elements and the like by melting a polymerwhile flowing a nitrogen gas into an extruder, and other methods arelisted.

[0028] The thickness of a polymerizing vessel after sealing a monomermixture into the polymerizing vessel affects controllability of apolymerization reaction. In general, when the thickness is increased,production amount increases, however, heat removal in polymerizationbecomes difficult and control of a polymerization tends to be lost, andthe polymerizing vessel may be broken, in some cases. When the thicknessis small, control of a polymerization is easy, however, the amount ofmaterials which can be polymerized at one time decreases. Therefore,thickness may be appropriately set depending on conditions of thefacility and the like. From the standpoints of controllability of apolymerization reaction and productivity, it is desired that thethickness in polymerization is set at about 2 cm to 5 cm.

[0029] For heating in a gas phase polymerization, general heatingfurnaces utilizing vapor and an electric heater may be advantageouslyutilized. However, when temperature distribution occurs inpolymerization, composition distribution occurs in a copolymer to lowerthe transparency of the copolymer in some cases. Therefore, higher gasvolume in a heating furnace is desirable, also to smoothly remove heatin polymerization and not to lose control of polymerization.

[0030] The methacrylic copolymer excellent in heat resistance of thepresent invention is suitable for optical elements such as illuminationcovers, automobile tale lamp parts, information recording mediumsubstrates, light connectors, pick up lenses mounted on photoelectronappliances utilizing semiconductor laser, projector lenses and the like,because of excellent heat resistance.

[0031] The present invention will be illustrated specifically based onexamples and comparative examples below. In examples, heat distortiontemperature was measured according to JIS-K7207, and “total lighttransmittance” and “haze” and “YI (yellow index)” were measuredaccording to JIS-K7105. The polymerization peak temperature was measuredunder condition of pasting the tip of thermocouple to the surface ofpolymerizing vessel.

EXAMPLE 1

[0032] In a vessel equipped with a stirring apparatus, 82% by weight ofMMA, 12% by weight of αMS and 6% by weight of MAH were mixed and stirredso that the total weight was 3 kg, and to this mixed solution having atotal weight of 3 kg was added 2700 ppm of n-octylmercaptane and 900 ppmof a polymerization initiator [manufactured by NOF Corp., trade name:Perbutyl O, 10 hour half life temperature: 72° C.], to prepare a monomermixture.

[0033] This monomer mixture was charged into a polymerizing vessel inthe form of bag having an inner size of 32 cm square constituted of amulti-layer film composed of the innermost layer which is a polyethylenefilm of 100 μm, the intermediate layer which is an oxygen shieldingmembrane (trade name: Vobron SE, manufactured by Nippon SyntheticChemical Industry Co., Ltd.) having an oxygen permeability of 2.4×10⁻¹⁵(mol·m⁻²·s⁻¹·Pa⁻¹) made-of an ethylene-vinyl alcohol copolymer film of12 μm and the outer layer which is a polyethylene terephthalate film of100 μm, and a purified nitrogen gas was bubbled for 50 minutes at a rateof 100 ml/min. to control the concentration of oxygen dissolved andremaining to 10 ppm or below, then, the vessel was sealed to give a bag.This bag had a thickness of 4 cm. This bag was placed in an air heatingfurnace, and polymerization was conducted at 72° C. The polymerizationpeak was observed 27 hours after placing into the heating furnace. Thepolymerization peak temperature was 114° C. After 30 hours passed, thebag was removed, then, polymerization at 130° C. was conducted for 2hours. The resulted copolymer had foaming, however, peeling off of themulti-layer film showed no problem.

[0034] The resulted copolymer was coarsely ground by a grinder, andpelletized with an extruder equipped with a vent while flowing apurified nitrogen gas from a hopper into a barrel, and this pellet wasinjection-molded to obtain a molded plate having a thickness of 4 mm.This molded plate had a heat distortion temperature of 118° C., a totallight transmittance of 92.0%, a haze of 0.6 and a YI value of 1.9, andthe resulted methacrylic copolymer had reduced yellowing, and excellenttransparency and heat resistance.

EXAMPLE 2

[0035] In a vessel equipped with a stirring apparatus, 85% by weight ofMMA, 9% by weight of αMS and 6% by weight of MAH were mixed and stirredso that the total weight was 15 kg, and to this mixed solution having atotal weight of 15 kg was added 3000 ppm of n-octylmercaptane, 1100 ppmof a polymerization initiator (Perbutyl O) and 50 ppm of apolymerization initiator [manufactured by NOF Corp., trade name: PerhexaMC, 10 hour half life temperature: 85° C., 100° C.], to prepare amonomer mixture.

[0036] This monomer mixture was charged into a polymerizing vessel inthe form of bag having an inner size of 70 cm×60 cm constituted of atwo-layer film composed of the outer layer which is an oxygen shieldingmembrane (trade name: Vobron SE, manufactured by Nippon SyntheticChemical Industry Co., Ltd.) having an oxygen permeability of 1.5×10⁻¹⁵(mol·m⁻²·s⁻¹·Pa⁻¹) made of a polyvinyl alcohol film of 14 μm and theinner layer which is a polypropylene film of 40 μm, and a purifiednitrogen gas was bubbled for 50 minutes at a rate of 100 ml/min. tocontrol the concentration of oxygen dissolved and remaining to 10 ppm orbelow, then, the vessel was sealed to give a bag. This bag had athickness of 4 cm. This bag was placed in an air heating furnace, andpolymerization was conducted at 72° C. The polymerization peak wasobserved 19 hours after placing into the heating furnace. Thepolymerization peak temperature was 110° C. After 22 hours passed, thebag was removed, then, polymerization at 130° C. was conducted for 2hours. The resulted copolymer had foaming, however, peeling off of themulti-layer film showed no problem.

[0037] Thereafter, the same procedure as in Example 1 was conducted toobtain a molded plate having a thickness of 4 mm. This molded plate hada heat distortion temperature of 120° C., a total light transmittance of92.0%, a haze of 0.3 and a YI value of 1.8, and the resulted methacryliccopolymer had reduced yellowing, and excellent transparency and heatresistance.

EXAMPLE 3

[0038] In a vessel equipped with a stirring apparatus, 86% by weight ofMMA, 6% by weight of αMS, 3% by weight of St and 5% by weight of MAHwere mixed and stirred so that the total weight was 15 kg, and to thismixed solution having a total weight of 15 kg was added 3000 ppm ofn-octylmercaptane and 800 ppm of a polymerization initiator (PerbutylO), to prepare a monomer mixture. Thereafter, the same procedure as inExample 2 was conducted to obtain a bag having a thickness of 4 cm.

[0039] This bag was placed in an air heating furnace, and polymerizationwas conducted at 72° C. The polymerization peak was observed 28 hoursafter placing into the heating furnace. The polymerization peaktemperature was 110° C. After 30 hours passed, the bag was removed,then, polymerization at 130° C. was conducted for 2 hours. The resultedcopolymer had foaming, however, peeling off of the multi-layer filmshowed no problem.

[0040] Thereafter, the same procedure as in Example 1 was conducted toobtain a molded plate having a thickness of 4 mm. This molded plate hada heat distortion temperature of 115° C., a total light transmittance of92.0%, a haze of 0.3 and a YI value of 1.5, and the resulted methacryliccopolymer had reduced yellowing, and excellent transparency and heatresistance.

EXAMPLE 4

[0041] In a vessel equipped with a stirring apparatus, 86% by weight ofMMA, 6% by weight of αMS, 3% by weight of St and 5% by weight of MAHwere mixed and stirred so that the total weight was 7.5 kg, and to thismixed solution having a total weight of 7.5 kg was added 3000 ppm ofn-octylmercaptane, 1150 ppm of a polymerization initiator (Perbutyl O)and 55 ppm of a polymerization initiator (Perhexa MC) to prepare amonomer mixture. Thereafter, the same procedure as in Example 2 wasconducted to obtain a bag having a thickness of 2 cm.

[0042] This bag was placed in an air heating furnace, and polymerizationwas conducted at 72° C. The polymerization peak was observed 16 hoursafter placing into the heating furnace. The polymerization peaktemperature was 114° C. After 19 hours passed, the bag was removed,then, polymerization at 130° C. was conducted for 2 hours. The resultedcopolymer had foaming, however, peeling off of the multi-layer filmshowed no problem.

[0043] Thereafter, the same procedure as in Example 1 was conducted toobtain a molded plate having a thickness of 4 mm. This molded plate hada heat distortion temperature of 115° C., a total light transmittance of92.0%, a haze of 0.3 and a YI value of 1.5, and the resulted methacryliccopolymer had reduced yellowing, and excellent transparency and heatresistance.

EXAMPLE 5

[0044] In a vessel equipped with a stirring apparatus, 77% by weight ofMMA, 13% by weight of St and 10% by weight of MAH were mixed and stirredso that the total weight was 8.0 kg, and to this mixed solution having atotal weight of 8.0 kg was added 2000 ppm of n-octylmercaptane and 480ppm of a polymerization initiator [manufactured by NOF Corp., tradename: Perloyl L, 10 hour half life temperature: 62° C.], to prepare amonomer mixture. Thereafter, according to the same procedure as inExample 2, a purified nitrogen gas was bubbled for 90 minutes at a rateof 100 ml/min. to control the concentration of oxygen dissolved andremaining to 1 ppm or below, then, a bag having a thickness of 2 cm wasobtained.

[0045] This bag was placed in an air heating furnace, and polymerizationwas conducted at 60° C. The polymerization peak was observed 15 hoursafter placing into the heating furnace. The polymerization peaktemperature was 95° C. After 22 hours passed, the bag was removed, then,polymerization at 130° C. was conducted for 2 hours. The resultedcopolymer had foaming, however, peeling off of the multi-layer filmshowed no problem.

[0046] Thereafter, the same procedure as in Example 1 was conducted toobtain a molded plate having a thickness of 4 mm. This molded plate hada heat distortion temperature of 116° C., a total light transmittance of92.0%, a haze of 0.3 and a YI value of 1.8, and the resulted methacryliccopolymer had reduced yellowing, and excellent transparency and heatresistance.

EXAMPLE 6

[0047] In a vessel equipped with a stirring apparatus, 81% by weight ofMMA, 3% by weight of αMS, 8% by weight of St and 8% by weight of MAHwere mixed and stirred so that the total weight was 8.0 kg, and to thismixed solution having a total weight of 8.0 kg was added 2000 ppm ofn-octylmercaptane, 150 ppm of terpinolene and 480 ppm of apolymerization initiator [manufactured by NOF Corp., trade name: PerloylL, 10 hour half life temperature: 62° C.], to prepare a mixture.Thereafter, polymerization was conducted in the same procedure as inExample 4. The polymerization peak was observed 17 hours after placinginto the heating furnace, and the copolymer was removed after 22 hourspassed, then, polymerization was conducted at 130° C. for 2 hours. Thepolymerization peak temperature was 90° C. The resulted copolymer hadfew foaming, and peeling off of the multi-layer film was easy.

[0048] Thereafter, the same procedure as in Example 1 was conducted toobtain a molded plate having a thickness of 4 mm. This molded plate hada heat distortion temperature of 115° C., a total light transmittance of92.0%, a haze of 0.3 and a YI value of 1.5, and the resulted methacryliccopolymer had reduced yellowing, and excellent transparency and heatresistance.

EXAMPLE 7

[0049] Polymerization was conducted in the same manner as in Example 5except that 150 ppm of terpinolene was added. The polymerization peakwas observed 15 hours after placing into the heating furnace. Thepolymerization peak temperature was 84° C. The resulted copolymer hadless foaming as compared with the copolymer in Example 5, and peelingoff of the multi-layer film was easy.

[0050] Thereafter, the same procedure as in Example 1 was conducted toobtain a molded plate having a thickness of 4 mm. This molded plate hada heat distortion temperature of 115° C., a total light transmittance of92.0%, a haze of 0.3 and a YI value of 1.9, and the resulted methacryliccopolymer had reduced yellowing, and excellent transparency and heatresistance.

COMPARATIVE EXAMPLE 1

[0051] A molded plate having a thickness of 4 mm was obtained in thesame manner as in Example 1 except that a polymerizing vessel in theform of bag made of nylon having a thickness of 50 μm [oxygenpermeability: 2.55×10⁻¹³ (mol·m⁻²·s⁻¹·Pa⁻¹)] was used. This molded platehad a YI value of 7.0, indicating very strong yellowing.

COMPARATIVE EXAMPLE 2

[0052] A molded plate having a thickness of 4 mm was obtained in thesame manner as in Example 2 except that a polymerizing vessel in theform of bag made of nylon having a thickness of 100 μm [oxygenpermeability: 1.79×10⁻¹³ (mol·m⁻²·s⁻¹·Pa⁻¹)] was used. This molded platehad a YI value of 5.4, indicating very strong yellowing.

COMPARATIVE EXAMPLE 3

[0053] A molded plate having a thickness of 4 mm was obtained in thesame manner as in Example 5 except that a polymerizing vessel in theform of bag made of nylon having a thickness of 100 μm [oxygenpermeability: 1.79×10⁻¹³ (mol·m⁻²·s⁻¹·Pa⁻¹)] was used. This molded platehad a YI value of 5.8, indicating very strong yellowing.

COMPARATIVE EXAMPLE 4

[0054] A molded plate having a thickness of 4 mm was obtained in thesame manner as in Example 5 except that 98.5% by weight of MMA, 1% byweight of St and 0.5% by weight of MAH were used and the total weightwas 8 kg. This molded plate had a heat distortion temperature of 103° C.

COMPARATIVE EXAMPLE 5

[0055] A molded plate having a thickness of 4 mm was obtained in thesame manner as in Example 5 except that 60% by weight of MMA, 22% byweight of St and 18% by weight of MAH were used and the total weight was8 kg. This molded plate had a YI value of 8.0, indicating very strongyellowing.

COMPARATIVE EXAMPLE 6

[0056] A molded plate having a thickness of 4 mm was obtained in thesame manner as in Example 5 except that 76% by weight of MMA, 18% byweight of St and 6% by weight of MAH were used and the total weight was8 kg. This molded plate had a haze of 3.1, indicating very strongturbidity.

COMPARATIVE EXAMPLE 7

[0057] A molded plate having a thickness of 4 mm was obtained in thesame manner as in Example 5 except that 72% by weight of MMA, 13% byweight of St and 15% by weight of MAH were used and the total weight was8 kg. This molded plate had a YI value of 5.1, indicating very strongyellowing.

[0058] The compositions and physical property values and the like inExamples 1 to 7 and Comparative Examples 1 to 7 are summarized in theflowing Table 1. TABLE 1 EX. 1 EX. 2 EX. 3 EX. 4 EX. 5 EX. 6 EX. 7Composition Methyl methacrylate (wt. %) 82 85 86 86 77 81 77α-Methylstyrene (wt. %) 12 9 6 6 0 3 0 Styrene (wt. %) 0 0 3 3 13 8 13Maleic anhydride (wt. %) 6 6 5 5 10 8 10 Terpinolene 0 0 0 0 0 0.0150.015 (parts by weight per 100 parts by weight of monomer mixture) Molarratio (1) 1.66 1.25 1.56 1.56 1.23 1.25 1.23 Physical Property ValueHeat distortion temperature (° C.) 118 120 115 115 116 115 115 Totallight transmittance (%) 92.0 92.0 92.0 92.0 92.0 92.0 92.0 Haze (%) 0.60.3 0.3 0.3 0.3 0.3 0.3 YI value 1.9 1.8 1.5 1.5 1.8 1.5 1.9 Comp. Comp.Comp. Comp. Comp. Comp. Comp. EX. 1 EX. 2 EX. 3 EX. 4 EX. 5 EX. 6 EX. 7Composition Methyl methacrylate (wt. %) 82 85 77 98.5 60 76 72α-Methylstyrene (wt. %) 12 9 0 0 0 0 0 Styrene (wt. %) 0 0 13 1 22 18 13Maleic anhydride (wt. %) 6 6 10 0.5 18 6 15 Terpinolene 0 0 0 0 0 0 0(parts by weight per 100 parts by weight of monomer mixture) Molar ratio(1) 1.66 1.25 1.23 1.88 1.15 2.83 0.81 Physical Property Value Heatdistortion temperature (° C.) — — — 103 — — — Total light transmittance(%) — — — — — — — Haze (%) — — — — — 3.1 — YI value 7.0 5.4 5.8 — 8.0 —5.1

[0059] As described above, according to the present invention, amethacrylic copolymer having reduced yellowing and excellenttransparency and heat resistance can be obtained by using a materialhaving low oxygen permeability as a material of polymerizing vessel.Further, the copolymer obtained according to the present invention hasreduced yellowing and shows excellent transparency. From the copolymerof the present invention, an optical element having excellent heatresistance, reduced yellowing and excellent transparency is obtained.

1. A process for production of methacrylic copolymers, characterized bycharging a monomer mixture comprising from 70 to 95% by weight of methylmethacrylate, from 0 to 15% by weight of α-methylstyrene, from 0 to 20%by weight of styrene and from 2 to 15% by weight of maleic anhydride ata molar ratio of the sum of α-methylstyrene and styrene to maleicanhydride laying within the range of 1.0 to 2.5 into a polymerizingvessel containing at least one polymeric membrane layer having an oxygenpermeability of 1.5×10⁻¹³ (mol·m⁻¹·s⁻¹·Pa⁻¹) or below, sealing thepolymerizing vessel, and polymerizing the monomer mixture under heatingin a gas phase.
 2. The process for production of methacrylic copolymersaccording to claim 1 wherein the monomer mixture comprises from 80 to95% by weight of methyl methacrylate, from 3 to 15% by weight ofα-methylstyrene, from 0 to 8% by weight of styrene and from 2 to 8% byweight of maleic anhydride at the molar ratio of the sum ofα-methylstyrene and styrene to maleic anhydride laying within the rangeof 1.0 to 2.5.
 3. The process for production of methacrylic copolymersaccording to claim 1 wherein the mixture was obtained by further addingfrom 0.0001 to 0.1 part by weight of a terpenoid compound per 100 partsby weight of the monomer mixture.
 4. The process for production ofmethacrylic copolymers according to claim 1 wherein a polymerizingvessel in the form of bag is used as the polymerizing vessel.
 5. Amethacrylic copolymer obtained by the production process according toclaim
 1. 6. An optical element composed of the methacrylic copolymeraccording to claim 5.